diff --git a/src/prolog/machine/system_calls.rs b/src/prolog/machine/system_calls.rs
index b4857b59..0f02d141 100644
--- a/src/prolog/machine/system_calls.rs
+++ b/src/prolog/machine/system_calls.rs
@@ -960,6 +960,8 @@ impl MachineState {
                                 let mut chars = String::new();
 
                                 for addr in addrs {
+                                    let addr = self.store(self.deref(addr));
+
                                     match Number::try_from((addr, &self.heap)) {
                                         Ok(Number::Fixnum(n)) => {
                                             match u32::try_from(n) {